Abstract

We have studied velocity-selective resonances in the presence of a uniform magnetic field and shown how they can be used for rapid, single-shot assessment of the ground state magnetic sublevel spectrum in a cold atomic vapor. Cold atoms are released from a magneto-optical trap in the presence of a small bias magnetic field (≈300 mG) and exposed to a laser field comprised of two phase-locked counterpropagating beams connecting the two ground state hyperfine manifolds. An image of the expanded cloud shows the velocity-selected resonances as distinct features, each corresponding to specific magnetic sublevel, in a direct, intuitive manner. We demonstrate the technique with both 87Rb and 85Rb, and show the utility of the technique by optically pumping into particular magnetic sublevels. The results are shown to agree with a theoretical model, and are compared to traditional Raman spectroscopy.

Figures (7)

a) Relevant energy levels for the Raman transitions. Magnetic sublevels are split by the Larmor precession frequency, ωL. The polarization configuration is lin⊥lin. The energy scale is exaggerated for clarity. The detuning, Δ, is measured from the D2 line. b) The possible transitions for Δm=0 (black), 1 (red), and -1 (blue). As discussed in the text, the ±2 orders are weak. Numerical values are shown for 85Rb.

Two techniques for phase-locking. (a) uses a triple-passed AOM. The deflection caused by the AOM occurs out of the plane of the figure, while the multiple-passing occurs in the plane as drawn. (b) uses an EOM to impose two sidebands. Only one of the sidebands is resonant by making the sidebands asymmetric.

Images of the expanded atom cloud as a function of angle between the Raman beam direction and the magnetic field for both 87Rb and 85Rb. We show cross-sections for 87Rb to indicate the labeling scheme. Odd numbers are derived from Δm=±1 transitions; Even numbers are from Δm=0 transitions. Arrows in the 87Rb image mark the locations of the 1/e points of the Gaussian cloud. Images and plots have been normalized to this distribution.

a) Cross-section amplitude as a function of Raman pulse duration. Fits are to a phenomenological model to extract the resonant two-photon Rabi frequency. The steady-state values are different because these measurements were not made on an unpolarized sample. b) Calculated results for our pulse parameters. The relative oscillation frequencies match the experiment very well. Oscillations in the experiment dephase more quickly due to time-varying magnetic fields and nonuniform laser intensity profiles.

Sublevel spectra recorded using traditional Raman spectroscopy (red circles) with the VSR technique (blue line). Fits to Gaussian profiles are shown as black solid lines. The widths of the Raman transitions for m≠0 are larger than those for m=0 due to time-varying magnetic fields. The integrated areas under the curves agree very well (see text).

Evolution of VSR images as a function of optical pumping pulse duration (left) and cross-sections (right) for three different cases. The images are montages of increasing pulse duration from 0 to 200 µs pulses. These are fluorescence images, shown in inverse palette for visibility. a) 87Rb pumped to m=0; b) 85Rb pumped to m=0; c) 85Rb pumped to m=−2.